Automatic test equipment for checking printed circuit boards has long involved use of a "bed of nails" test fixture in which the circuit board is mounted during testing. This test fixture includes a large number of nail-like spring-loaded test probes arranged to make electrical contact under spring pressure with designated test points on the circuit board under test, also referred to as the unit under test or "UUT." Any particular circuit laid out on a printed circuit board is likely to be different from other circuits, and consequently, the bed of nails arrangement for contacting test points in the board must be customized for that particular circuit board. When the circuit to be tested is designed, a pattern of test points to be used in checking it is selected, and the corresponding array of test probes is configured in the test fixture. This typically involves drilling a pattern of holes in a probe plate to match the customized array of test probes and then mounting the test probes in the drilled holes on the probe plate. The circuit board is then mounted in the fixture superimposed on the array of test probes. During testing, the spring-loaded probes are brought into spring-pressure contact with the test points on the circuit board under test. Electrical test signals are then transferred from the board to the test probes and then to the exterior of the fixture for communication with a high speed electronic test analyzer which detects continuity or lack of continuity between various test points in the circuits on the board.
It is commonly known that electrostatic discharges (ESD) can cause serious damage to semi-conductor devices. In recent years it has been learned that circuits are exposed to potentially damaging electrostatic charges in the course of being checked on automatic test equipment. Electrostatic charges can build up on a printed circuit board as it is being moved from one location to another due to, for example, the triboelectric effect, or if static charges build up from handling the board prior to its being tested. Also, during testing it is common for vacuum to be used to exert the force on the UUT to engage it against the bed of nails, in which case air rushes through the test fixture during application of the vacuum as well as when the vacuum is released. The friction of air particles moving against the fixture surfaces and across the board is a source of electrostatic charge build up.
Several approaches have been used in the past in an attempt to deal with the electrostatic charges that develop in a test fixture during use. One such approach is described in U.S. Pat. No. 4,814,698 to St. Onge, et al. uses an anti-static substance having a certain critical resistivity applied to areas of a vacuum test fixture. Such vacuum test fixtures are in a class of test fixtures often referred to as customized or wired test fixtures in which the test probes are individually wired to separate interface contacts for use in transmitting test signals from the test probes to the external electronically controlled test analyzer. Such customized wire fixtures are particularly useful in testing circuit boards with complex arrangements of test points and low volume production boards where larger and more complex and expensive electronic test analyzers are not practical.
In addition to customized wired test fixtures, a further class of test fixtures is the so-called grid-type fixture in which translator pins contact random patterns of test points on the board and transfer test signals to interface pins arranged in a grid pattern in a receiver. In this grid type tester, fixturing is generally less complex than in the customized wired test fixtures, but with a grid system, the grid interfaces and test electronics are substantially more complex and costly. The present invention is directed toward the prevention of ESD damage to the more costly grid type fixtures.
A typical grid fixture contains test electronics with a huge number of switching circuits connecting test points in a grid base to corresponding test circuits in the electronic test analyzer. In one embodiment of a grid tester as many as 80,000 test probes are used in top and bottom test fixtures for communicating with complex grid test electronics in which as many as 80,000 corresponding switches are used. During a test cycle in such a grid tester, large probe plates are moved together quickly, compressing the plates together on opposite sides of the UUT. Air rushing away from the probe plates and over the board surfaces can cause a build up of electrostatic charges. Placing the UUT on the fixture also can cause static charges to develop. During testing a large number of metal contact pins on the test probes make sudden contact with switch cards containing expensive integrated circuit modules for the switching circuitry. Such grid fixtures if unprotected for the effects of ESD can suffer costly damage as a result of the build up of such electrostatic charges and a resulting sudden discharge during testing.
The present invention provides a ground plane arrangement for dissipating accumulated charges in a grid tester so that ESD effects are prevented during the test cycle.